David Allingham from Northern Ireland crossed the finish line ahead of Luke Mossey by 0.166 seconds in the British Superstock event held at Cadwell Park.
The result sees Allingham now sitting 42 points clear of his nearest rival, English rider Ryan Vickers.
This marks the eighth victory for Allingham this season as he enters the final three rounds of the British Superbike Championship campaign.
Scott Swann, who started from pole position, finished fourth after Billy McConnell passed him for the third-place spot.
Northern Ireland's Glenn Irwin did not complete the opening race on Monday but recorded a 14th place finish in the concluding event of the bank holiday weekend.